Chapter 23, Problem 23.21P

Interpretation Introduction

Interpretation:

The mechanism for electrophilic aromatic substitution of pyridine at the $4$ position on the ring, using ${\text{E}}^{\text{+}}$ as a generic electrophile is to be drawn. Whether the corresponding arenium ion intermediate is more or less stable than the one in Equation $\text{23-31b}$ is to be determined, using resonance arguments.

Concept introduction:

Aromatic compounds are resistant to electrophilic addition. Instead, these compounds show electrophilic substitution to preserve their aromaticity. Furan, pyrrole, and pyridine are heterocyclic aromatic compounds. These compounds undergo electrophilic aromatic substitution reactions, same as benzene. But the reaction rates for these compounds can be faster or slower than benzene, characterizing their rings as either activated or deactivated. The first step of these reactions is electrophilic addition of the electrophile ${\text{E}}^{\text{+}}$ to the aromatic carbon, which forms the arenium ion intermediate. The more stable arenium ion intermediate leads to form the major product of this substitution reaction.